Miller 570-T "Hi-Fidelity" AM Tuner
1942
J. W. Miller Company,
5917 S. Main Street,
Los Angeles, California, USA

    In 1937, William N. Weeden published an article, in the February 1937 issue of Electronics magazine, describing a radically different TRF tuner using band pass circuitry to accomplish Hi-Fidelity audio on the AM broadcast band.  Later the J. W. Miller Company took Weedens circuit, made improvements on it, and marketed it as the Miller model 570 tuner and model 570A receiver.  These two models are one of the few TRF receivers commercially produced that incorporated AVC and a 10 KC filter.  Miller marketed these units mainly for the use of professional broadcast monitoring by radio station engineers but it was also used by audiophile enthusiasts.  Over time, Miller produced at least three versions of the 570 in which the only thing that mostly varied in each version was the dial and the tubes that were used.  The tuner featured above, the 570-T, is of the third version of Miller's 570 series.  It's audio responce is comprable to that of the 1953 Miller 595 "Hi-Fidelity" crystal tuner but it gets more stations then what the 595 can.  Today these units are considered to be quite rare although not impossable to aquire.  I aquired this tuner from an ebay auction in February of 2006.

Tube Line Up:
6SK7...1st. R.F. Amplifier
6SK7...2nd. R. F. Amplifier
6SF7...Detector/AVC
6C5...1st. Audio
6C5...2nd. Audio
6E5...Tuning Indicator

Measurements:
Height...8.75 inches
Width...15 inches
Depth...12.25 inches

Frequency Ranges:
BC Band...550 kHz - 1800 kHz

Power Source:
AC...110 Volts

    Information and schematics for this tuner can be found at "John's Radio Pages" web site by clicking on the link below.

Schematic and Information

    This is a blue print of the early version 570 which is the same as my set except it uses 5 and 6 pin tubes instead of octal tubes.

  Miller Quality Products Catalog
No. 40
Page 12

A Wide-band Tuner
By
W. N. Weeden
Electronics - February 1937
Pages 19 - 21

Designed for reception of programs from local stations, this TRF tuner delivers sufficient output to drive push-pull 2A3 tubes, passes sidebands of approximately 7.5 to 10 kc., and has sufficient adjacent channel selectivity

    This tuner was designed to meet the requirements of those who wish high fidelity radio reception from high power local stations. Many people have been in the market for a high fidelity receiver for some time but have not felt like paying the money for a one-microvolt, all-wave receiver when they wished to restrict their listening to not over a half dozen stations delivering several millivolts of signal. The tuner may also be used as a monitor for broadcast programs by advertisers or any others interested only in local programs.
    The selective element is made up of 4 tuned circuits in pairs of coupled or band pass filter circuits. The coupling of negative mutual inductance and capacitance insures a sensibly constant bandwidth, which can be varied simply by changing the capacity of the coupling condenser. A diode detector is fed from this selective system through an untuned transformer. The detector is followed by a single stage of triode a-f amplification, which provides sufficient voltage output to load up push-pull 2A3 tubes through a proper coupling transformer.
    The tuned circuits are overcoupled and exhibit the usual curve of peaks separated by a dip at resonance. This dip amounts to about 2 db and is not considered serious, considering the variation in response of the usual loud speaker. A 10,000-cycle filter made up of a tuned trap is located in the output of the first a-f tube. In this particular receiver, the trap can be removed at will, but in general it is advisable to have it in the circuit. On any good night in the New York suburbs the beat notes between stations makes it quite necessary to use the trap. With a 7,500-cycle side band an attenuation of 60-70 db is necessary to eliminate monkey chatter.
    The diagram shows that the coupling impedance consists of a negative mutual inductance, a resistance and capacity. The negative mutuals must be wound by hand. For checking coil inductance and condenser matching, a simple test method is to make the coil or condenser section under test part of the oscillatory circuit of a simple oscillator. The simplest battery-operated, unshielded oscillator will serve although it is preferable to include a variable condenser of 15 to 25 micro-microfarads in parallel to the main oscillator tuning capacitor. With this auxiliary condenser set at midscale the main control may be varied until a beat is produced with a broadcast station (picked up by the receiver) at approximately the frequency desired for the test.
    By cutting the various sections of the tuning condenser into the oscillatory circuit, it is possible to retune to zero beat by the auxiliary condenser and the discrepancy in capacity may be readily noted. By bending plates it is possible to bring the capacity back into line. As most midget condensers are of the straight-line capacity type the capacity per dial degree can be calculated roughly. Coils may be checked by a similar method. The accuracy of alignment by this method depends upon the patience of the operator. An accuracy of 0.1 per cent is not unattainable.
    The interesting circuit using the negative mutual inductance was first described by E. A. Uehling in Electronics, September 1930, and was used commercially in the well-known 10-A wide-band receiver of Western Electric. That receiver, however, used a square law detector so that modulation peaks affected the AVC action. The present tuner utilizes a linear detector, the AVC operates only on the carrier and is unaffected by the modulation. When the AVC is removed for manual r-f gain control, modulation does affect the input to the tuning indicator (6G5 on the diagram) so that a slight wavering of the shadow is seen. This does not bother the tuning, however.
    Additional data on the negative mutual inductance coupling circuit may be found in the "Radio Engineering Handbook," 2nd Edition, page 158, ~ in Wireless World, February 18, 1931, and in Radio Engineering, December 1936.
    Because this tuner was to be operated by an engineer several controls were placed in it, which would not be necessary or desired, perhaps, for more general use. For example the slight amount of AVC secured from the detector is fed back to the first stage and a switch makes it possible to remove this voltage and to control the r-f gain of the receiver manually. Selective fading seems to be tolerated with somewhat more enjoyment when the receiver is not under AVC. A tone control has been included in the receiver but has never been used by the engineer for whom the set was made. When static is so bad that a tone control is necessary, receiving is no fun anyhow. For general use the tone control might as well not be included.
    The second r-f stage must be operated with sufficient bias, say 7 to 9 volts to prevent amplitude distortion when it is supplying 60 volts peak at 100 percent modulation to the diode circuit, the impedance of which is about 50,000 ohms. The audio frequency stage is conventional except that its grid is connected to one-half the diode load to improve the modulation capability of the detector. If insufficient output is secured, the a.f. may be connected across the entire diode load. When 20 volts are applied to the diode, approximately 100 volts (rms) will be applied between the grids of a push-pull amplifier when fed by a 2-1 transformer from the first a-f stage.
    There are two untuned transformers available. Both are replacement items and may be secured from jobbers or factory branches handling these lines. One is a Fada unit, which gives somewhat greater output above 1200 kc., and the other is a Stromberg Carlson unit, which is slightly more efficient below 650 kc. The Fada unit is more compact. In such a receiver it is necessary to reduce as far as possible any chance of noise entering the circuits. Grid and plate leads must be short and direct; separate leads should run from each brush of the gang condenser, a single point on the chassis should represent ground for diode and a-f stages. This point should be near the diode.
    After antenna, ground and power supply have been connected, the 1st r-f screen voltage should be adjusted (by moving clip on Electrad Truvolt voltage divider) to 125 and the cathode to plus 3. Then the 2nd 6D6 screen voltage should be set at 125 to 150 (depending on signal strength) and its cathode to plus 8 volts.
    Then, with micro-ammeter or magic eye, proceed to align or trim the gang condenser (Wholesale Radio Service YH9705) at 1400 to 1500 kc.
    After aligning carefully, the screen voltages may need to be reset so that with AVC operating, the weakest of the local network stations will impress 8 to 10 volts across the diode input (80-100 microamperes through the 100,000 ohm load).
    If the AVC fails to hold down the signal with greatest field strength to 20 volts, readjust screen-grid voltages, and if no satisfactory compromise can be made-shift the AVC connection to point B from A, thus impressing twice the control voltage on the first r-f tube, at the expense of a slight increase in harmonic distortion. Although these adjustments may sound tedious, it will probably be found that they can be accomplished in less time than that necessary to read this portion of the paper.
    Although the diagram shows the 10,000-cycle filter (Philco) in the first a-f plate circuit, experience has shown that a Stromberg Carlson filter is somewhat more satisfactory. It is higher in impedance and should be placed in the diode load. The connecting leads must be very short.
    With coils as described and coupled with condensers of 0.05 f the bandwidth is approximately 20 kc. and when shunted by an additional 0.01 f this bandwidth becomes approximately 15. The coils are special and to the best of the writer's knowledge may be obtained only from J. W. Miller Co., 5917 South. The loud speaker presents a problem. There are several fine speakers now available which are satisfactory up to 6000 cycles or so, but the writer has found that it takes one of the few double-unit speakers to really make a wide-band tuner show up its capabilities. The natural resonance of the cone of the speaker should be in the neighborhood of 30 cycles, if possible. This calls for one of the 18-inch or similarly large dynamic types with plenty of field excitation. Many listeners state that they find little advantage to extending
    Main St., Los Angeles. While it is desirable to check the inductance and the capacity of the several units, it is true that reasonable variation between values of the several units will not cause great departure from symmetry of resonance curve nor appreciable change in fidelity.
    A Yaxley switch may be employed to change not only the bandwidth but in the narrower position to insert the whistle filter as well. This refinement is unnecessary unless using loud speakers, which are flat to 10,000 cycles or better. With most single speakers, even the so-called high-fidelity units, sharp cut off usually takes place between six and eight thousand cycles, and little or no difference will be noted between a 7,500 and 10,000 cycle band width.
    This tuner has been designed to operate with an efficient signal collector, because wide-band reception is worthless unless the signal to noise ratio is high. The antenna should be at least 20 feet away from wiring, other antennas, metal roofs, gutters, etc., and as long as possible -at least 75 to 150 feet. A good ground plus a noise-reducing leadin or transmission line with a transformer at each end are very desirable. A metal cabinet to prevent direct pick-up by leads or condenser stators will be desirable.
The loud speaker presents a problem. There are several fine speakers now available which are satisfactory up to 6000 cycles or so, but the writer has found that it takes one of the few double-unit speakers to really make a wide-band tuner show up its capabilities. The natural resonance of the cone of the speaker should be in the neighborhood of 30 cycles, if possible. This calls for one of the 18-inch or similarly large dynamic types with plenty of field excitation.
    Many listeners state that they find little advantage to extending the audio range into the high frequencies unless the low end is extended at the same time. Thus a small baffle with a cut-off near 100 cycles is distinctly not good enough for faithful reproduction. The several acoustical systems recently developed-such as the labyrinth of Mr. Olney, the resonant pipes of RCA Victor, the resonating cones of Philco or the "bass reflex" principle of Hugh Knowles of Jensen-may be effectively employed. In the latter system, use is made of ports in the speaker cabinet, which are approximately resonant in the extreme low frequency range of the cabinet so that in relation to the cubic content, the phase of the back radiation of the cone is reversed.

This web page was last updated: May 10, 2007